1. Field of the Invention
The present invention relates to a light-emitting film, a light-emitting device, and a production method thereof. More specifically, the present invention relates to a light-emitting film and a light-emitting device which can be utilized in LEDs and inorganic ELs, and a production method thereof.
2. Description of the Related Art
Recently, the advances in research of light-emitting devices having high-luminance emission have been remarkable, with light-emitting devices based on various operational principles being developed. Examples include LEDs and LDs which emit light by the recombination of electrons and holes injected in semiconductor pn junctions formed of high-quality crystals; and inorganic ELs which emit light by electrical field excitation by applying a high electric field to an insulating phosphor thin film and exciting a light-emitting center in the phosphor thin film with hot electrons. Further examples include organic ELs obtained by laminating a light-emitting layer, an electron transporting layer, and a hole transporting layer formed of organic molecules or a polymer thin film, which cause exciton emission that is localized in the organic molecules by the recombination energy between the injected electrons and holes. Among these, although LEDs and organic ELs, which are capable of high-luminance emission by a DC drive, have been effectively incorporated into people's daily life, the fact remains that there is a need for even higher luminance and improved energy consumption. In addition, there is also a strong need for technological development of light-emitting devices which can be produced more easily and which have high durability.
Currently, in response to the above needs, the following light-emitting devices have been developed. In the “Journal of Crystal Growth 117 (1992) 1035-1039”, light-emitting devices having a MIS (Metal-Insulator-Semiconductor) structure and a MISIM (Metal-Insulator-Semiconductor-Insulator-Metal) structure have been reported. The light-emitting film is a semiconductor layer using ZnS:Ag,Cl for the donor-acceptor-pair recombination. The Ag/Zn atom number ratio is 30 to 50 ppm, which is very low. Thus, the light-emitting device is AC-driven, and the emission starting voltage is 35 Vrms. The emitted luminance at 50 Vrms is 30 cd/m2. Further, the journal article contains no description regarding the resistivity of the light-emitting film. In addition, a MOCVD method is used for the production of such a light-emitting film, and the deposition rate is 20 nm/min.
Japanese Patent Publication No. H06-097704 discloses a light-emitting device in which, in order to perform MIS structure hole injection emission stably, the I layer is formed from two or more hole injection high-resistance insulating layers. Low-resistance single crystal ZnS or epitaxial crystal film is used in the light-emitting portion. Further, the single crystal ZnS does not contain an addition element, and the resistivity is 1 to 5 Ωcm. The external quantum efficiency is about 0.08%.
FIG. 20 illustrates a schematic diagram of a typical conventional DC drive-distributed EL device described in “Phosphor Handbook” (Phosphor Research Society, 1987, page 326). In the DC drive-distributed EL device, a DC current pathway is formed by coating CuxS 52, which has higher electroconductivity than ZnS, on granular ZnS crystals 60 by a forming treatment. This “forming treatment” is a treatment in which current is flowed with a transparent electrode 58 as the positive and an electrode 59 as the negative to form a high-resistance region about 1 μm in thickness, in which Cu+ ions are depleted on the transparent electrode 58 side by making the mobile Cu+ ions move to the electrode 59 side so that the CuxS is segregated. Further, the CuxS coating the granular ZnS crystals also acts as a carrier source. Specifically, since CuxS has a higher electroconductivity than ZnS, an electric field concentrates on the CuxS which has segregated in a differential region when voltage is applied. As a result, the electrons of the valence band of the CuxS are trapped in the donor level of the base ZnS, and the holes are trapped in the acceptor level. It is believed that light is emitted by the recombination of the electrons trapped in the donor level with the holes trapped in the acceptor level.
However, in the above-described conventional light-emitting device, the light-emitting film cannot itself be provided with both light-emitting capabilities and a desired resistivity. As a result, the emission starting voltage increases, and it is more difficult to obtain high-luminance emission. Further, to realize a conventional light-emitting device, complex productions processes, such as a MOCVD method, a single crystal production method and its resistivity control method, and an epitaxial deposition method, are required, meaning that production is not simple.